I started to create a simple sample by using Blank project template.

Next, I added an interface called ICustomService which inherits IMobileComponentService. The snippet code below shows how the interface is declared in ICustomService.cs file in the Core project.

using Intersoft.Crosslight.Mobile;

namespace CustomService.Core.Custom
{
    public interface ICustomService : IMobileComponentService
    {
        void DoSomething();
    }
}

In the SimpleViewModel class, ShowToast function is modified by resolving CustomService via IoC (injection of Container) technique. If customService is not null, invokes DoSomething() method.

private void ShowToast(object parameter)
{
    //this.ToastPresenter.Show("You entered: " + this.NewText);
    //this.GreetingText = this.NewText;

    ICustomService customService = ServiceProvider.GetService<ICustomService>(); //resolve CustomService via IoC (injection of Container)
    if (customService != null)
    {
        customService.SetOwner(this); //needed for Android platform only
        customService.DoSomething();
    }
}

Next, we need to register the custom service in the AppInitializer class, specifically in the InitializeServices method.

public void InitializeServices(IApplicationHost appHost)
{
    ServiceProvider.AddService<ICustomService, CustomiOSService>();
}

Please note that we need to repeat this step for each platform (Android, iOS, Windows Phone, and Windows StoreApp).

Last, implement DoSomething method for each platform. In the sample, what DoSomething doing is very easy, it shows "Custom Service" alert.

using CustomService.Core.Custom;
using Intersoft.Crosslight.iOS;
using MonoTouch.UIKit;

namespace CustomService.iOS.Custom
{
    public class CustomiOSService : IOSMobileComponentServiceBase, ICustomService
    {
        public void DoSomething()
        {
            UIAlertView alert = new UIAlertView("Custom service", "Custom Service", null, "OK", null);
            alert.Show();
        }
    }
}

The sample can be downloaded in https://onedrive.live.com/download?resid=A29317908CEA783A%21408. Please let us know whether this help or not.